1. Field of the Invention
The present invention relates to an automatic document feeder for automatically feeding a plurality of documents, such as document sheets, to be copied.
2. Description of the Related Art
In an automatic document feeder, particularly in a recycle document handler, a mechanism is provided which prevents simultaneous feeding of a plurality of document sheets lying one upon another (hereafter referred to as a multiple feed). Nevertheless, a multiple feed of document sheets (originals) sometimes occurs.
There is a prior art for preventing a multiple feed when copies are made by using a recycle document handler (Japanese Patent Application Laying Open (KOKAI) No. 52-119940). In this prior art, when a set of original sheets has been copied, the number of document sheets presented for copying is stored in memory. This number is compared with the number of document sheets circulated for copying after the next set of document sheets has been copied. When these numbers differ, a multiple feed is indicated, and the document handler is shut down.
According to the prior art mentioned above, it is after a set of document sheets has been circulated that a decision can be made whether a multiple feed has occurred. Until then, copying of the document sheets is continued. A resulting problem is that many document sheets which are involved in a multiple feed are not copied. Above all, when duplex copying is performed, there is another problem that ineffective copying occurs in which front and back combinations of the contents of the document sheets are displaced with respect to each other by a multiple feed.
When the number of document sheets is counted and the sheets are copied according to the counted number, if a multiple feed occurs, this results in a wrong counting of the number of document sheets and the copying operation is controlled according to a wrong number of document sheets. If in this case duplex copying is performed on a number of copying paper from simplex document sheets, many copies are made by ineffective copying in which the front and back combinations of the contents of document sheets are displaced with respect to each other.